This invention relates to a process of and apparatus for separating air to produce oxygen-enriched gas.
Oxygen-enriched gas from air separation has many uses, for example as a breathing atmosphere. In some instances, the users are people suffering from respiratory ailments such as emphysema who need a readily-available oxygen supply in the home. In other instances, oxygen is used for high altitude breathing in aircraft.
A common requirement for breathing oxygen is that the source be lightweight. One approach has been to charge specifically designed equipment with liquid oxygen and use built-in means for vaporization on demand, but this system requires an external source of liquid oxygen and is not self-sufficient. Another approach is a pressure swing adsorption of nitrogen in a crystalline zeolite molecular sieve adsorbent, with the unadsorbed gas discharged as oxygen product.
One type of pressure swing adsorption system described in Wagner U.S. Pat. No. 3,430,418 employs four adsorbent beds arranged in parallel flow relationship with each bed proceeding sequentially through a multistep cycle. Since oxygen product discharge from a given bed is not continuous, the beds are arranged so that at least one of the four beds is always producing product oxygen-enriched gas discharged from the second end. In brief each bed employs an adsorption step in which at least one component of the feed gas mixture is selectively adsorbed from the feed gas introduced at the bed first end and the oxygen-enriched product gas is discharged from the second end of such bed. The previously described adsorption step is usually performed at the highest pressure of the process and is followed by a first depressurization step in which gas discharged from the bed at progressively lower pressure is used to perform other functions in the process, as for example repressurizing another previously purged bed and/or purging still another bed. This first depressurization step is usually in the same direction i.e. cocurrent, as the feed gas previously flowing through the bed during the adsorption step. After the first depressurization step a final depressurization step usually follows and is most commonly countercurrent to the gas flow during the preceding adsorption and first depressurization steps. During this step gas is usually released at the inlet end and contains desorbate. When depressurization is completed, a purge gas is usually introduced at the second end for countercurrent flow through the bed to desorb and sweep out the desorbate at the inlet end. When purging is completed the bed is repressurized with one component-depleted gas in preparation for return to the previously described adsorption step, and the cycle is repeated.
One disadvantage of the previously described type of pressure swing adsorption process is the complexity of the piping and multiple valving required to provide the necessary flow switching. Still another disadvantage is that the cycles are relatively long, resulting in large, heavy beds and equipment. By way of example, in one such system for air separation the total cycle time for each bed to complete the adsorption through repressurization sequence for air separation in a four bed system is about 4 minutes. This means that the production rate of oxygen-enriched product gas per pound of adsorbent (hereinafter referred to as "adsorbent productivity") is relatively low.
The aforedescribed four bed system is used to provide relatively high pressure and high purity (e.g. at least 90% 0.sub.2) oxygen product at substantially feed air pressure of up to about 100 psig. If lower pressure product is desired as with breathing oxygen, the three bed system described in Batta U.S. Pat. No. 3,636,679 is more suitable. In this system compressed feed air and product oxygen are simultaneously introduced at opposite ends to the same adsorbent bed, the latter gas being obtained from another bed being cocurrently depressurized. The flows are continued until the two beds are pressure equalized, whereupon only the feed air flow is continued for further repressurization prior to release of oxygen product gas from the opposite end.
Further savings in equipment cost may be achieved by the two bed system described in McCombs U.S. Pat. No. 3,738,087. In the McCombs system an increasing pressure adsorption step is employed with feed air introduced to the first end of a partially repressurized adsorbent bed at higher pressure than initially present in such bed. Nitrogen is selectively adsorbed in the bed and oxygen product gas is discharged from the bed second end. The feed air introduction, nitrogen adsorption and oxygen product gas discharge are at relative rates such that the pressure of the adsorbent bed rises from the intermediate pressure to higher pressure at the end of the step.
Notwithstanding these improvements, the previously described pressure swing adsorption systems (hereinafter broadly described as "PSA") have high power requirements and low adsorbent productivity for supplying breathing oxygen to the individual user. In order to change bed pressure in various PSA cycle steps, multiple valves and product gas manifolds are required to isolate individual beds from the rest of the system.
One possible approach to overcoming the previously enumerated disadvantages of multiple bed-relatively long cycle time PSA processes is the rapid pressure swing adsorption process (hereinafter broadly described as "RPSA"). In the RPSA system as for example described by P. Turnock ("The Separation of Nitrogen and Methane by Pulsating Flow Through a Fixed, Molecular Sieve Bed," Ph.D Thesis, University of Michigan, 1968), a single adsorption bed is provided comprising relatively small particles of adsorbent. The adsorbent particle size used by the prior art may, for example be between 40 and 80 mesh whereas with the afore-described multiple bed-relatively long cycle time PSA system the major dimension of individual particles may, for example be 1/16 inch or larger pellets. As used herein, mesh size ranges refer to U.S. standard screen commonly used for sizing small particles. By way of example, "between 40 and 80 mesh" or "-40 +80 mesh" means particles in a size range which pass through a 40 mesh screen and are retained on an 80 mesh screen.
The adsorbent is a crystalline zeolite molecular sieve of at least 5 angstroms apparent pore size, as for example calcium zeolite A ("5A") described in Milton U.S. Pat. No. 2,882,243 and sodium zeolite X ("13X") described in Milton U.S. Pat. No. 2,882,244. Compressed feed air is introduced to the first end of the adsorbent bed and nitrogen is selectively adsorbed from the feed air and oxygen-enriched gas is continuously discharged from adsorbent bed second end into a product conduit with a product surge tank upstream of the discharge valve.
In the RPSA system the small adsorbent particles provide the necessary flow resistance to operate the process whereas in PSA this flow resistance is minimized to reduce pressure drop in the absorbent bed. The aforedescribed flow continues for a predetermined period which will hereinafter be referred to as the "feed air introduction period" and the oxygen-enriched gas discharged from the single bed during this period will be termed the "product gas."
Following the feed air introduction period the feed valve is closed and an exhaust valve in a reverse outward flow conduit joining the inlet end is opened. During the exhaust (reverse outward flow) period nitrogen-depleted (or oxygen-enriched) gas within the adsorbent bed flows in the reverse direction towards the first or inlet end. This gas sweeps nitrogen gas towards the first end after such gas has been desorbed from the adsorbent by pressure reduction i.e., the pressure differential between the gas in the bed during the feed air gas introduction period and the exhaust pressure. Flow reversal occurs in the adsorbent bed while product oxygen is being removed from the second end, and the flow reversal zone moves quickly from the first to the second end during exhaust. As will be explained hereinafter typical times for the feed air introduction period and the second or reverse outward flow period are relatively short and on the order of 0.1 to 10 seconds. Although not essential, RPSA systems often employ a flow suspension or time delay step between the feed air introduction and reverse outward flow, and during such period the feed inlet and exhaust valves are both closed but discharge of oxygen-enriched product gas is continued through the second end.
According to the previously referenced Turnock thesis, research work was done at Esso Research Laboratories on separating air with an RPSA system. Turnock states only that "--The parameters were studied over the following ranges: 20-50 percent feed time per cycle, 15-40 psig. feed gas pressure, 0.25-8.0 cps. cycling frequency and 20-200 standard cubic centimeter of product gas per minute. The feed capacity of the column at 30 psig. feed gas pressure ranged from 0.475 to 13.5 liters per minute for cycling frequencies of 0.25 and 7.6 cps., respectively. Product gas compositions for air feed to the column were as high as 99 mol percent oxygen. The higher compositions generally resulted at the lower product gas flow rates, the higher cycling frequencies, and the higher feed gas pressures.--" Unfortunately there is no direct information on the performance of the Esso single bed RPSA system, either in terms of product recovery (the percent of oxygen in the feed gas which is recovered as product at the second end) or the adsorbent productivity. However, it may be calculated that the product recovery for 99 mol percent oxygen could not have exceeded 1%, and there is no reason to believe that the experimenters achieved higher product recovery when producing lower purity oxygen. A product recovery of 1% is prohibitively low and not acceptable for commercial use even when the feed gas is unlimited as with air separation.
In any type of pressure swing adsorption system the investment cost is the sum of a function of the recovery (reflecting the compressor cost), plus the adsorbent productivity (reflecting the cost of the vessel(s) holding the adsorbent), and other minor items. In general the investment cost is most greatly influenced by the product recovery and this factor represents between 30% and 80% of the investment cost. In general by increasing the product recovery at any given pressure and product purity, one decreases the compressor cost and increases the cost related to the adsorbent holding vessel. Because of the dominating product recovery factor the aforementioned investment cost trade-off emphasizes the importance of relatively high product recovery processes. In addition to investment cost, the practioner must consider operating expense, i.e. power cost. Whereas the latter is unaffected by adsorbent productivity it is directly affected by product recovery and by feed compression ratio. It will be recognized that product recovery may be increased by increasing the feed pressure but the resulting increase in recovery is more than offset by the increase in compression ratio, and this results in an overall increase in power consumption.
Other researchers have discovered that by employing in a single bed a relatively short feed air introduction period such as 1 second, a feed air suspension period such as 0.5 second and an exhaust period of about 4 seconds, oxygen recovery as high as 15%, and 0.6 scfh. contained 0.sub.2 per lb. adsorbent (productivity) may be achieved.
There are however several important disadvantages of the single bed RPSA system as compared with multiple bed PSA systems for high purity oxygen production. The delta loading, a measure of adsorbent efficiency on a per cycle basis, is lower because of the smaller pressure swing at the second or product end of the RPSA bed. The oxygen recovery is lower in the RPSA system, primarily because of the lower bed utilization and a higher average pressure in the RPSA bed during the exhaust. Finally, the power consumption is higher in the RPSA system because of the greater pressure drop through the bed.
A disadvantage of the single bed RPSA system when used to supply breathing oxygen is a relatively long "start-up," i.e. the time required for a system to reach desired product flow rate and purity from the moment the air compressor is started. It will be appreciated that this is an extremely important consideration for emphysema victims and in high altitude breathing.
An object of this invention is to provide an improved rapid pressure swing system for separating air to produce high purity oxygen suitable for breathing.
Another object is to provide an improved RPSA air separation system to produce high purity oxygen suitable for breathing, which is lighter and more compact than heretofore proposed systems producing breathing oxygen.
Still another object is to provide an improved RPSA air separation system to produce high purity oxygen suitable for breathing, in which the product recovery and adsorbent productivity are in the aggregate substantially higher than attainable in a single bed RPSA system.
A further object is to provide an improved RPSA air separation system to produce high purity oxygen suitable for breathing, which uses less power than a single bed RPSA system or two bed PSA system.
Another object is to provide an improved RPSA system with shorter start-up time.
Other objects will be apparent from the ensuing disclosure and appended claims.